1. Field of the Invention
This invention relates generally to antimicrobial peptides and specifically to derivatives of the antimicrobial cationic peptide, bactenecin.
2. Description of Related Art
In 1981, the self-promoted uptake hypothesis was first proposed to explain the mechanism of action of polycationic antibiotics in Pseudomonas aeruginosa. According to this hypothesis, polycations interact with sites on the outer membranes of Gram-negative bacteria at which divalent cations cross-bridge adjacent lipopolysaccharide molecules. Due to their higher affinity for these sites, polycations displace the divalent cations and, since the polycations are bulkier than the divalent cations, cause structural perturbations in the outer membrane. These perturbations result in increased outer membrane permeability to compounds such as the .beta.-lactam antibiotic nitrocefin, the eukaryotic non-specific defense protein lysozyme and to hydrophobic substances. By analogy, molecules accessing this pathway are proposed to promote their own uptake.
It has been clearly demonstrated that the outer membranes of Gram-negative bacteria are semipermeable molecular "sieves" which restrict access of antibiotics and host defense molecules to their targets within the bacterial cell. Thus, cations and polycations which access the self-promoted uptake system are, by virtue of their ability to interact with and break down the outer membrane permeability barrier, capable of increasing the susceptibility of Gram-negative pathogenic bacteria to antibiotics and host defense molecules. Hancock and Wong demonstrated that a broad range of such compounds could overcome the permeability barrier and coined the name "permeabilizers" to describe them (Hancock and Wong, Antimicrob. Agents Chemother., 26:48, 1984). While self-promoted uptake and permeabilizers were first described for P. aeruginosa, they have now been described for a variety of Gram-negative bacteria.
Over the past decade, non-specific defense molecules have been described in many animals, including insects and humans. One subset of these molecules have in common the following features: (a) they are small peptides, usually 15-35 amino acids in length, (b) they contain 4 or more positively charged amino acid residues, either lysines or arginines, and (c) they are found in high abundance in the organisms from which they derive. Several of these molecules have been isolated, amino acid sequenced and described in the patent literature (e.g., cecropins: WO 8900199, WO 8805826, WO 8604356, WO 8805826; defensins: EP 193351, EP 85250, EP 162161, U.S. Pat. No. 4,659,692, WO 8911291). However, only limited amounts of these peptides can be isolated from the host species. For example, Sawyer, et al., (Infect. Immun. 56:693, 1988) isolated 100-200 mg of rabbit neutrophil defensins 1 and 2 from 10.sup.9 primed peritoneal neutrophils or lipopolysaccharide-elicited alveolar macrophages (i.e., the numbers present in a whole animal).
The gene for human defensin has been cloned and sequenced, but no successful expression has been demonstrated, as yet. Furthermore, production of these peptides using peptide synthesis technology produces peptides in limited amounts and is expensive when scaled up or when many variant peptides must be produced. Also, structural analysis is difficult without specific incorporation of .sup.15 N and .sup.13 C tagged amino acids which is prohibitively expensive using amino acid synthesis technology.
Recently, cationic peptides containing a disulphide bond forming a looped structure were identified (Morikawa et al., Biochim. Biophys. Res. Commun. 189:184, 1992; Simmaco et al., FEBS 324:159, 1993; Clark et al., J. Biol. Chem. 269:10849, 1994). One member of this group, bactenecin (i.e., dodecapeptide), is a twelve amino acid peptide isolated from bovine neutrophils (Romeo et al., J Biol. Chem. 263:9573, 1988). Bactenecin is the smallest known cationic antimicrobial peptide. Two cysteine residues form a disulphide bond to make bactenecin a loop molecule. Bactenecin was previously found to be active against Escherischia coli and Staphylococcus aureus, and strongly cytotoxic for rat embryonic neurons, fetal rat astrocytes and human glioblastoma cells (Radermacher et al., J. Neuroscience Res. 36:657, 1993).
There is a need to develop peptides having a broad range of potent antimicrobial activity against a plurality of microorganisms, including gram negative bacteria, gram positive bacteria, fungi, protozoa, viruses and the like. To that end, the small size and structure of bactenecin present an opportunity to identify derivatives of the polypeptide which are effective as therapeutics for microbial pathogens.